The present invention relates to devices for detecting small quantities of oxygen which may be present in other gases, by the use of galvanic cells. More particularly, the invention relates to a galvanic cell oxygen detector which is capable of detecting minute quantities of oxygen in trace levels which are not detectable by prior art galvanic cell devices.
U.S. Pat. No. 3,223,597, Hersch, discloses a galvanic cell construction which includes a chemically reducing anode, a non-conductive thin porous electrolyte-retentive diaphragm in intimate contact with at least one surface of the anode, a cathode comprising a thin porous conductive sheet in intimate contact with the surface of the diaphragm opposite to the surface in contact with the anode, and an aqueous electrolyte contained in the porous diaphragm. The pores of the cathode are only partially filled with electrolyte, and the cathode is chemically nonreactive to the electrolyte, the anode being incapable of evolving hydrogen upon being circuited with the cathode. The total volume of electrolyte is less than the combined total pore volume of the cathode, diaphragm and anode, so that only a minor portion of the pores of the cathode become wetted while a major portion of the pores of the cathode never become filled. This construction results in an electrode assembly whereby only the surface of the cathode which is in intimate contact with the diaphragm is wetted, and the top surface and major portion of the pores of the cathode remain substantially dry.
U.S. Pat. No. 4,085,024, Lawson, discloses essentially the same electrode assembly as the Hersch patent, using a particular construction technique for interconnecting the elements of the cell and sealing it inside an oxygen-free envelope.
The foregoing galvanic cell constructions are critically dependent upon the concentration and quantity of electrolyte contained within the cell. Further, the sensitivity of the cell degrades with time, as the water content of the electrolyte becomes dissipated from the cell. If careful design and construction practices are followed, reliable results in detecting oxygen to about 30 parts per billion can be achieved for short periods of time, at test gas flow rates of 10-20 cubic centimeters (cc) per minute. However, this degree of accuracy is difficult to achieve because the time required for equilibrium is often longer than the time the cell remains stable.
In addition to the sensitivity variations which may be caused by varying electrolyte concentrations, there are a number of other factors which reduce the sensitivity of such galvanic cells. These variations affect the output voltage from the galvanic cell, which under idealized conditions should be directly proportional only to the oxygen content in the gas undergoing tests. For example, variations in the temperature of the device itself, or of the gas undergoing tests, will introduce output voltage variations which tend to obscure the desired output voltage signal. Further, electrochemical variations within the cell, thermal EMFs, outgasing from the wall of the cell and/or the materials within the cell, and gas pumping through the walls of the cell and/or the materials in the cell will all generate output voltage variations, which variations may be considered "noise" which obscures the desired output voltage readings. These sources of "noise" voltage may generate currents in the cell in the range of 10.sup.-8 amps per degree Centigrade change in temperature. Therefore, the prior art galvanic cells have a practical lower limit in the measurement of oxygen quantities present in the test gas, which lower limit is approximately the "noise" current generated by the cell itself.
There is a need for an oxygen sensor of the galvanic cell type which can overcome the problems of the prior art sensors, and successfully provide reliable and accurate measurements of oxygen content to a level well below about 30 parts per billion. Furthermore, there is a need for an oxygen sensor wherein sensitivity to temperature variations can be eliminated, or greatly reduced, thereby enabling the reliable use of the sensor over a wider range of environmental conditions. Finally, there is a need for an oxygen sensor having improved sensitivity over a longer useful life, being less dependent upon electrolyte concentrations in the sensor during its useful life.